在 Django 视图中使用 select_related 会增加查询时间 [英] Using select_related in django view increases query times

问题描述

我想尝试在 Django 视图中使用 select_realted 来提高性能.我比较了使用 select_realted 之前和使用之后的结果.

I thought of trying to use select_realted in Django view to boost the performance. And I compare the results before using select_realted and after using it.

虽然我看到大量查询减少了，但时间却在增加.所以我不确定是否在每个视图中使用 select_related 或不使用它们.

Although I see the reduction of significant number of queries, the time however rises. So I am not sure whether to use select_related in every view or not use them.

我只想知道什么时候用，什么时候不用.

I just want to know when to use them and when not to.

我的观点是:

之前:

class ProductAPIView(ListAPIView):
    permission_classes = [AllowAny]
    serializer_class = ProductSerializer
    queryset = Product.objects.all()

之后:

class ProductAPIView(ListAPIView):
    permission_classes = [AllowAny]
    serializer_class = ProductSerializer
    #queryset = Product.objects.all()

    queryset = Product.objects.select_related('merchant','brand','collection','sub_category')
    pagination_class = CustomPagination

我的模型:

class Product(models.Model):        
    merchant = models.ForeignKey(Seller,on_delete=models.CASCADE,blank=True,null=True)
    category = models.ManyToManyField(Category, blank=False)
    sub_category = models.ForeignKey(Subcategory, on_delete=models.CASCADE,blank=True,null=True)
    brand = models.ForeignKey(Brand, on_delete=models.CASCADE)
    collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
    featured = models.BooleanField(default=False)  # is product featured?

select_related 使用前后的图片.

The images before and after the use of select_related.

之后:

这里我们可以看到查询次数减少到 124，但时间增加到 227.那么，我应该使用 select_related 吗?什么时候用，什么时候不用??

Here we can see that the number of queries are reduced to 124, but the time is increased to 227. So, should i be using select_related? When to use it or when not to use it??

推荐答案

Django中的select_related其实就是SQL中的JOIN.
select_related 将仅在一个查询中获取您的相关字段以及您的对象.
例如，假设您有一个 Book Model 并且您有以下代码:

the select_related in Django is actually JOIN in SQL.
select_related will fetch your related fields as well with your object in only one query.
for example imagine if you have a Book Model and you have this following code:

# Django will execute another query for getting author name
book = Book.objects.get(id=1)  
author = book.author.name
# 2 queries executed  

在这个例子中，你只执行了一个查询，但是为了获得 author.name Django 将执行另一个查询.但是如果你使用 select_related Django 也会在一个查询中得到作者.

In this example you have executed only one query but for getting the author.name Django will execute another query. But if you use select_related Django will get the author too in just one query.

# Django won't execute another query for getting author name
book = Book.objects.select_related('author').get(id=1)  
author = book.author.name
# 1 query executed  

请注意，如果您不需要它们，则无需在 select_related 中选择所有相关对象.
请查看此链接 在 stackoverflow 中了解 select_related 和 prefetch_related 之间的区别以及何时使用它们.
另请查看此链接以了解有关 n + 1 的更多信息查询问题.

Note that you don't need to select all of your related objects in select_related if you don't need them.
Please check out this link in stackoverflow to know what's the differences between select_related and prefetch_related and when to use them.
also check out this link to know more about n + 1 query problems.

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